Background: This study focused on undergraduate L2 students' performance in written historical reasoning, particularly written historical contextualization, before and after participating in a historical reasoning course. The Content and Language Integrated Learning course was designed using a cognitive apprenticeship model and was based on principles likely to facilitate students' written historical reasoning. Methods: Conducted as a quasi-experimental study, students in an experimental condition received explicit instruction in historical contextualization and other features of historical reasoning, while those in the control group participated in a version of the course without a focus on historical contextualization. Students' historical reasoning was measured based on their argumentative document-based writing. Findings: Students' in both the experimental and control groups significantly improved in all of the areas of historical reasoning that we measured. There was not a significant difference between the groups in the area of historical contextualization, but a further qualitative analysis demonstrated traces of the instructional approach in students' writing. Unexpectedly, students in the experimental group were significantly better than the control group in terms of writing claims. Possible explanations for this finding are discussed. Contributions: This study makes contributions in terms of operationalizing and measuring written historical contextualization, particularly among L2 undergraduate students.